Formate Adsorption onto Thin Films of Rutile TiO2 Nanorods and Nanowires

We evaluate the applicability of silicon prisms to infrared (IR) spectroscopic investigations of the oxide/electrolyte interface in the attenuated total reflection (ATR) configuration. Using formic acid as a probe adsorbate, a comparison is done between a rutile nanowire film supported on Si and a rutile nanorod film supported on ZnSe. The nanowires were 2 nm in diameter and were deposited directly on Si by chemical bath deposition, whereas the nanorods were deposited on ZnSe by solution casting of an aqueous dispersion prepared from a commercial powder. The lower penetration depth of the evanescent wave for silicon was compensated by a higher internal surface area of the corresponding nanowire film. Advantageously, much higher solution concentrations can be used in the case of the Si prism without a significant contribution of solution species to the IR spectrum. Furthermore, the high chemical stability of Si opens up the possibility of performing experiments in highly acidic aqueous solutions. Upon formate adsorption at pH 3.5, a pair of intense IR bands was observed in the wavenumber range where antisymmetric v(as)(COO) vibrations are expected, namely at 1537 and 1592 cm(-1) on nanorod films and at 1544 and 1586 cm(-1) on nanowire films. The relative band intensities are different for nanorod and nanowire films. While the bands at 1537/1544 cm(-1) are assigned to formate adsorbed on the (110) face, forming the bridging bidentate (mu) structure, those at 1592/1586 cm(-1) are tentatively attributed to formate adsorbed at low coordination adsorption sites at the nanocrystal edges. Bands corresponding to the carboxylate symmetric stretching and the HCO deformation were also observed.